1. Field of the Invention
This invention pertains to method and apparatus for completion of wells by gravel packing. More specifically, improved gravel placement outside the gravel-packing screen is provided.
2. Description of Related Art
Mechanical exclusion is the most common method of preventing sand or other particles from entering a well as fluids are produced from or injected into the well. Various devices, called screens or slotted liners, having openings small enough to exclude particles when fluid flows into the well through the devices have been used. The placement of particles larger than formation particles between the screen or slotted liner and the formation is called gravel packing. The "gravel" size, normally from about 0.01 inch to several times this size, is selected to prevent movement of grains of formation material through the gravel pack. The size of openings in the screen is selected to prevent movement of gravel through the screen. The gravel is placed by pumping a slurry into the well after the screen is in place.
Gravel packing of wells began in the water well industry; it has been widely adopted in the petroleum industry. The pack may be placed in open hole or inside perforated casing. There are advantages and disadvantages of each type completion, but the cased-hole gravel pack is more common in oil and gas wells.
Placement of the gravel to form an effective filter in the well that has highest conductivity for fluids but prevents solid entry is a critical step in the gravel-packing operation. It is also important that no voids exist in the gravel pack, because formation solids may then flow through the screen and cause "sand-up" of the well. It is especially important in cased-hole gravel packs that gravel be placed between the perforations and the screen. Lack of gravel to protect the screen from the high velocity fluids entering through perforations can lead to early failure of the gravel pack from erosion of the screen opposite a perforation.
The fluid used to form the gravel slurry to be pumped into the well may be water-based, oil-based, an emulsion or a foam. Polymers may be used to increase the viscosity of the water or oil. If viscous fluid is used, the gravel is pumped at high concentration (in the range of 10 pounds per gallon) in the slurry, in a process called slurry packing. If low viscosity fluid is used, gravel concentrations in the range of 1 pound per gallon of the gravel are normally pumped and the fluid is circulated into and out of the well, the gravel particles being filtered out and formed into a pack as fluid passes through the screen in the well. Fluid may also flow out the perforations during the packing process.
Different techniques are available for flowing the gravel into a well. In the reverse circulation technique, the slurry is pumped down the annulus outside tubing in the well, gravel is filtered out on the screen, and return fluid flows back to surface through the tubing. More commonly, the "cross-over" method is used, in which the slurry is pumped down the well inside tubing, under conditions of higher flow velocity to prevent bridging of the particles, until the slurry is near the screen. A "cross-over" tool then directs flow from inside the tubing to the annulus outside the screen. Also connected to the cross-over tool is the "wash pipe," which is a pipe inside the screen which transports the fluid with gravel removed. The return fluid then flows up the annulus outside the tubing. The cross-over tool may be mounted inside a packer which seals the annulus outside the tool, called a "cross-over packer."
In vertical wells, gravel-packing success is greater than 90%. However, many wells are drilled which are deviated from vertical--offshore from platforms and in the arctic from pads, for example. Angles of deviation where the wellbores intersect hydrocarbon reservoirs are often above 45 degrees, and may be as high as 90 degrees. In addition to the deviated wells, more recently there has been rapid growth in the number of horizontal wells, in which holes are drilled at near 90 degrees angle for substantial distances in hydrocarbon reservoirs. Horizontal wells and deviated wells are also drilled for remediation of contamination of ground water and for other environmental applications. The success of gravel packing in these deviated and horizontal wells has been significantly less than in vertical wells. The lower success is generally ascribed to the difficulty of obtaining uniform and complete gravel placement in the annulus outside the screen. Gravel tends to form mounds or dunes outside the screen during placement. Settling of incompletely packed gravel around the screen also leaves parts of the annulus void of gravel. The state-of-the-art of gravel packing in general, and gravel packing in deviated wells in particular, is reviewed in the recent monograph Sand Control, Society of Petroleum Engineers, Richardson, Tex., 1992. As pointed out in Chap. 8 of this reference, at angles above about 60 degrees, transport and settling of gravel around screens requires special considerations.
One of the proposed solutions to the problem of gravel packing of highly deviated wells was to place cups on the wash pipe. The cups were proposed to decrease the tendency for sand to form dunes outside the screen. However, even with the cups, fluid can still enter or leave the annulus between the cups on the wash pipe. U.S. Pat. No. 4,046,198 discloses use of wash pipe (or "stinger") of increased diameter so as to increase flow resistance in the annulus between the wash pipe and the screen and minimize formation of dunes outside the screen. While this latter technique can increase placement efficiency of gravel, as shown in the monograph Sand Control on page 49, at high well deviation angles placement efficiency is still below 100 per cent. This failure to pack the annulus outside the screen with gravel can lead to gravel settling to the bottom of the annulus and gravel pack failure in deviated wells.
U.S. Pat. No. 5,165,476 proposes method and apparatus for increasing the efficiency of gravel placement in wells by restricting flow into the upper part of the screen during circulation of gravel slurry into the well. The restriction is then removed after gravel has been placed. The area of screen open for flow during gravel placement is fixed.
There is a long-felt need for method and apparatus to increase the efficiency of gravel placement outside screens devices in wells, particularly in wells where the screen device is at a high angle from vertical. The method should allow formation of a uniform gravel pack around the screen, the gravel filling the annulus outside the screen over the entire length of the screen.